Hi. Is the new unit or the existing one? Is your fuel gas coming from an outside source (not refinery fuel gas)?
You should investigate if the fouling is due to entrained amines and pipe corrosion products. That should be fixed up stream of the DCU. You must ensure you have not exceeded the vapor velocity limits for the main fuel gas KO pot in the DCU to prevent entrainment fouling. Some plants use SS piping for the FG system but this is $$$$.
How heavy is the tail end? Are you experiencing burner tip fouling? Consider individual coalescers at each furnace especially if you have Ultra-low NOx burners.